Wood Vinegar Liquid

Wood vinegar, Pyroligneous acid, also called wood acid, is a dark liquid produced by the destructive distillation of wood and other plant materials. The principal components of wood vinegar are acetic acid, acetone and methanol. It was once used as a commercial source for acetic acid. In addition, the vinegar often contains 80-90% water along with some 200 organic compounds.

Wood vinegar is a byproduct from charcoal production. It is a liquid generated from the gas and combustion of fresh wood burning in airless condition. When the gas is cooled, it condenses into liquid. Raw wood vinegar has more than 200 chemicals, such as acetic acid, formaldehyde, ethyl-valerate, methanol, tar, etc. Wood vinegar improves soil quality, eliminates pests and controls plant growth, but is slightly toxic to fish and very toxic to plants if too much is applied. It accelerates the growth of roots, stems, tubers, leaves, flowers, and fruit. In certain cases, it may hold back plant growth if the wood vinegar is applied at different volumes. A study shows that after applying wood vinegar in an orchard, fruit trees produce increased amounts of fruit. Wood vinegar is safe to living matters in the food chain, especially, insects that help pollinate plants.

Type of wood
Broad-leaved trees such as Japanese oak, beech and white oak are considered good raw material. However, Quercus acutissima, a broad-leaved tree as well, has a low component concentration.

Second to broad-leaved trees, pine, cedar, hinoki cypress, Japanese larch and bamboo are often employed in farm products. Of these, the component concentration in pine is rather high.

Different woods have different component ratios. For example, acetic acid - a representative component of wood vinegar - is found in broad-leaved trees in quantities of 3.5-4.0% compared to 1.4% in bamboo.

Extraction temperature
The duration of extraction is limited to the period in which white smoke is emitted, and does not include the period of initial smoke. Thus, extraction should begin when the temperature of the charcoal (earthen) kiln exhaust port is 80-84‹C and end when the temperature is 110-120‹C. If the smoke turns blue, the substance turns into fibrin-decomposed tar, thereby deteriorating the germicidal effect of the product.

Immediately

After extraction, a chemical reaction (polymerization) continues to take place with the wood vinegar components. Thus, the substance should be allowed to stand for a period of 3-6 months until the components settle.

Wood vinegar that contains foreign elements or appears dirty when a small amount is placed in a glass container is poor quality. A good quality product has a clear yellowish-brown to reddish-brown tone. The color should be similar to the pleasing hue of black tea, beer or wine.

The wood vinegar should not have an offensive odor. Wood vinegar that has a strong, pungent odor is a synthetic product containing a substance such as glacial acetic acid to adjust the pH.

The pH value should be around 3. Higher extraction temperatures result in higher acidic values.

The specific gravity of wood vinegar should be around 1.015 at a liquid temperature of 15‹C (when Japanese oak is carbonized in an earthen kiln). Wood vinegar extracted at higher temperatures has a higher specific gravity. Please note that the specific gravity varies greatly when a device other than an earthen kiln, such as an iron kiln or combustion furnace, is used.

You can also identify good wood vinegar using chemicals. The wood vinegar is acceptable if the liquid remains transparent at the neutral point, and if the odor remains the same as the initial odor without abnormalities. The chemical test must prove that the liquid is the cell fluid (sap) of a young tree.

The crude wood vinegar, bio-oil, is very complex solution contained with at least 200 constituents. Some of those components can be prone to such complex reaction as oxidization and polymerization. Physicochemical characteristics of wood vinegar produced from different sources even between producing batch in the same source have very high variability. It will bring to the inconsistency on the utilization efficiency. Therefore, if wood vinegar is to be a future source of natural chemicals production with consistence activity, an effective separation method must be developed to generate semi-purified bioactive components. Several methods such as standing, filtering, distillation and solvent extraction have been developed for semi-purified and classified components. In order to obtain more specific and consistent property of product, the wood vinegar may be fractionated into semi-purified product. These could be achieved by several techniques such as sedimentation, filtration, chromatography, distillation and solvent extraction.

Standing method
This is used to prepare wood vinegar from crude bio-oil. It is simplest and most highly efficient method that keep crude bio-oil standing in a container. When left standing, the unstable constituents in raw wood vinegar are oxidized or polymerized to precipitate, suspend or adhere to the inner wall of the container. The thin oily film on the surface of the liquid and transparent wood vinegar separate on the middle phase must be discarded and the suspended as well as precipitated matters are filtered to produce transparent wood vinegar. When the standing and filtering processes are repeated several times, stable, transparent wood vinegar is obtained. While the standing method requires a long time, it is easy and inexpensive compared to other methods and good results are assured. In practice, wood vinegar is easily separated from whole bio-oil and a viscous oligomeric lignin-containing fractions setting at the bottom by standing at least 3 months.

Filtration
Filter like filter paper is used to remove the precipitated as well as suspended matters. The filtering of freshly recovered bio-oil cannot fully remove unwanted constituents, causing the appearance rely on filtering method. To obtain transparent wood vinegar, combining the filtering with the standing method is necessary. In the filtering process, the oil and suspended matters in wood vinegar gradually clog the filter paper or filter, lengthening the filtering time. In order to prevent this, frequent changes of the filter paper or filter are necessary. The appearance of suspended matter in transparent wood vinegar after future standing means that unstable constituent remains requiring further filtering.

Column chromatography
The principle of column chromatography is that substances are separate based on their different adsorption capabilities on stationary phase. Large polar compounds are contained in wood vinegar. In general, highly polar molecules are easily adsorbed in the stationary phase, while weak polar molecules are not. Thus, the process of column chromatography involves adsorption, desorption, re-adsorption, and re-desorption. Silica gel is commonly used as the stationary phase, and an eluent is selected based on the polarity of compounds from wood vinegar. Paraffin eluents, such as hexane and pentene, are used to separate aliphatic compounds. Aromatic compounds are usually eluted with benzene or toluene. Some other polar compounds are obtained by elution with methanol or other polar solvent.

Distillation
Distillation is a common separation technology in the chemical industry. This method separates the components successively according to their different volatilities, and it is essential for the separation of liquid mixtures. In general, there are two distillation systems i.e. the normal pressure and reduced pressure distillation. In both systems, compounds are separated by mean of the deference boiling points

The carbonizing wood vinegar has water content of as high as 80–90% with a rather small difference in boiling point between the remaining 10% of organic matters. Therefore, the boiling of wood vinegar starts below 100°C under atmospheric pressure, and then the distillation continues up to 250–280°C, whereupon 35–50% of residue is left.

Distillation method is quite effective to concentrate wood vinegar and also to remove substance with particularly low and high boiling points. However the distillation process cannot entirely remove unwanted polymer. It is more practical to use this method after unwanted polymer in crude vinegar removed by standing method., However, it should be carefully because the heating to boil sample may be induce the oxidation and polymerization in which bring to losing bioactivity of any components.

Solvent extraction
Liquid–liquid extraction method so called solvent extraction, involves the selective transfer of a substance from one liquid phase to another. Usually, an aqueous solution of the sample is extracted with an immiscible organic solvent. Thus the solute A distributed between an aqueous and an organic solvent:

where square bracket denote concentration and KD is known as the equilibrium distribution of partition coefficient which is independent of total solute concentration. It should be note that constant temperature and pressure are assumed and that A must exist in exactly the same from in both phases. Equilibrium is established when the chemical potentials (free energies) of the solute in the two phase are equal and is usually achieve within a few minutes by vigorous shaking. The value of KD is a reflection of the relative solubility of the solute in the two phases.

Solvent extraction or liquid–liquid extraction has been introduced for semi-purification active compounds from wood vinegar mixture. This technique is used for the separation of compounds with different partition or relative solubility between the 2 solvent phases. Commonly, an aqueous solution of the sample is extracted with an immiscible organic solvent. By selecting appropriate polarity of the solvents for extraction, such as hexane, diethyl ether, ethyl acetate, acetone, water etc., the desired products may be obtained. In order to obtain the higher purity of bioactive compounds from wood vinegar, Oasmaa et al. suggested that step-by-step extraction on the basis of polarity order may be employed. Some reports showed that phenolic compounds and organic acids were extracted from wood vinegar using ethers and dichloromethane. However, they found that a considerable amount of the high polarity and volatile compounds have been lost because of co-evaporation of the compounds at solvent drying step.

In another phenomenon, synergistic function was characterized to be the mode of action of wood vinegar. The most researchers have long suggested that phenolic and organic acid were active component. However, the new report in 2011 presented other unidentified components has high possibility to be active compounds. The highly classified efficiency method to identified components is very necessary. It will bring to the right answer about what is important bioactive compound of wood vinegar and then it might be used as biochemical markers to quantify wood vinegar quality.